Diagnostic equipment for an exhaust gas cleaning apparatus

ABSTRACT

A diagnostic equipment for an exhaust gas cleaning apparatus installed for an engine, comprising a misfire detector which detects the misfire of the engine, and a secondary-air-system failure detector which detects the failure of a secondary air system. An index corrector corrects a deterioration index calculated by a deterioration-index calculator, in accordance with the detected result of the detector. A deterioration decision unit decides if the diagnostic equipment has failed, by the use of the corrected deterioration index. In a case where the extent of the misfire or the like is severe, a decision interrupter interrupts the decision of the deterioration decision unit. Thus, even when the misfire of the engine or the failure of the secondary air system has occurred, the detection of the deterioration of a catalyst does not err. It is therefore avoided to erroneously replace the catalyst which has not deteriorated yet, or to run the engine in spite of the deterioration of the catalyst.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a diagnostic equipment for anexhaust gas cleaning apparatus installed in an engine system. Moreparticularly, it relates to a diagnostic equipment for an engine exhaustgas cleaning apparatus which employs a catalyst together with aso-called “UEGO sensor(universal exhaust gas oxygen sensor)” that servesto measure an air fuel ratio in a wide range of air-fuel-ratio values ora so-called “O₂-sensor” (oxygen sensor) that generates a binary outputon the basis of a sudden output change near a stoichiometric ratio(hereinbelow, both the sensors shall be collectively called an“air-fuel-ratio sensor”).

[0003] 2. DESCRIPTION OF THE RELATED ART

[0004] A well-known apparatus for cleaning the exhaust gas of an enginehas a catalyst and an air-fuel-ratio feedback controller. The catalystis incorporated in an exhaust pipe part for the purpose of eliminatingHC (hydrocarbons), NO_(x) (nitrogen oxides) and CO (carbon monoxide)which are contained in the exhaust gas, The air-fuel-ratio feedbackcontroller is a device which is disposed for the purpose of causing thecatalyst to demonstrate the function thereof satisfactorily, and whichexecutes a control so as to hold the air fuel ratio of the exhaust gasat a predetermined value, while measuring the air fuel ratio by the useof an air-fuel-ratio sensor mounted upstream of the catalyst.

[0005] In such an exhaust gas cleaning apparatus, when theair-fuel-ratio sensor for measuring the air fuel ratio has undergonedeterioration in its performance, the air fuel ratio sometimes deviatesfrom the predetermined value, resulting in increase in the amounts ofthe noxious gas components contained in the exhaust gas. Moreover, theair fuel ratio may fall outside a ratio range in which the catalyst candemonstrate its performance, and the elimination efficiency (conversionefficiency) of the catalyst for the noxious gases may lower. On theother hand, when the catalyst itself has undergone deterioration in itsperformance, the conversion efficiency thereof for the noxious gaseslowers in spite of the control of the air fuel ratio into the ratiorange in which the catalyst can demonstrate its performance. In thismanner, the deterioration of the performance of the air-fuel-ratiosensor or the catalyst results in increasing the amounts of the noxiousgases which are emitted into the atmosphere. Therefore, a diagnosticequipment for the exhaust gas cleaning apparatus has been contrived inorder to diagnose the performances during the drive of a vehiclefurnished with the cleaning apparatus and to give warning to the driverof the vehicle against the deteriorations. By way of example, thediagnostic equipment is so constructed and operated that air-fuel-ratiosensors are respectively disposed upstream and downstream of thecatalyst, and that an air-fuel-ratio feedback control is executed on thebasis of, at least, the output of the air-fuel-ratio sensor locatedupstream of the catalyst, while the deterioration of the catalyst isdetected on the basis of the output of the air-fuel-ratio sensor locateddownstream of the catalyst, etc. Such prior-art techniques are disclosedin the official gazettes of Japanese Patent Applications Laid-open No.91440/1990 and No. 286160/1991.

[0006] With the method wherein, during the air-fuel-ratio feedbackcontrol, the deterioration of the catalyst is detected on the basis ofthe output of the air-fuel-ratio sensor located downstream of thecatalyst, naturally the detection of the deterioration of the catalystis impossible while the air-fuel-ratio feedback control is at rest.Besides, in such a case where the catalyst has not been activated yet,the detection of the catalyst deterioration is highly liable to err. Itis therefore necessary to permit and inhibit the detection of thecatalyst deterioration in accordance with the operating conditions ofthe engine, for example, the revolution speed (revolutions per minute)and load thereof. These factors are considered also in the prior-arttechnique disclosed in the official gazette of Japanese PatentApplication Laid-open No. 91440/1990. Further, in a case where theair-fuel-ratio sensor located upstream of the catalyst has deteriorated,the detection of the catalyst deterioration is affected depending uponthe extent or content thereof. Therefore, it is sometimes necessary toinhibit a correction for a detected result and the detection of thecatalyst deterioration. In the prior-art technique disclosed in theofficial gazette of Japanese Patent Application Laid-open No.286160/1991, accordingly, the detection of the catalyst deterioration isinhibited when the upstream air-fuel-ratio sensor has deteriorated.

[0007] Meanwhile, when the engine has misfired in the combustion strokethereof, oxygen in air flows into an exhaust pipe together with unburntgas. In consequence, the air-fuel-ratio sensor located upstream of thecatalyst generates a spike signal indicating the lean exhaust gas, orthe air-fuel-ratio sensor located downstream of the catalyst generates asignal indicating leaner exhaust gas than the actual one. This poses theproblem that the accuracy of the detection of the catalyst deteriorationlowers. Also in a case where a secondary air system disposed forintroducing the air into the exhaust pipe has failed, the problem of thelowering of the detection accuracy is involved for such a reason thatthe air-fuel-ratio feedback control does not proceed normally. Thesedrawbacks are not considered in any of the prior-art techniques.

SUMMARY OF THE INVENTION

[0008] An object of the present invention is to provide a diagnosticequipment for an exhaust gas cleaning apparatus, in which the accuracyof the detection of the deterioration of an air-fuel-ratio sensor aswell as a catalyst is prevented from lowering even when the misfire ofan engine and the failure of a secondary air system have taken place.

[0009] According to the present invention, when the misfire of an enginehas been detected, a correction is made for a deterioration index whichexpresses the extent of the deterioration of a catalyst or anair-fuel-ratio sensor and which is calculated in the detection of thecatalyst or sensor deterioration, or the decision of the deteriorationstate of the catalyst or air-fuel-ratio sensor is interrupted during thedetection of the misfire. Besides, in case of the failure of a secondaryair system, the correction or interruption stated above is similarlydone.

[0010] Incidentally, the correction may well be made only in a casewhere frequence in the misfire is comparatively low or where the failureof the secondary air system is comparatively light. The decision of thedeterioration state of the catalyst or air-fuel-ratio sensor may well beinterrupted in a case where the frequence in the misfire is high orwhere the failure of the secondary air system is heavy. On thisoccasion, the deterioration state of the catalyst or air-fuel-ratiosensor cannot be decided. In general, however, the misfire of the engineand the failure of the secondary air system do not often occur, and sucha construction is satisfactory in many uses. With this contrivance, evenwhen the misfire of the engine or the failure of the secondary airsystem has occurred, the detection of the deterioration of the catalystor air-fuel-ratio sensor is possible to some extent without lowering theaccuracy thereof.

[0011] The construction of the present invention will be described moreconcretely below.

[0012] In the first aspect of performance of the present invention,there is provided a diagnostic equipment for an exhaust gas cleaningapparatus; the exhaust gas cleaning apparatus being directed toward anengine system furnished with an air-fuel-ratio controller which detectsan air fuel ratio of exhaust gas emitted from an engine and whichadjusts a quantity of fuel injection so as to hold the air fuel ratio ofthe exhaust gas at a predetermined value, and cleaning the exhaust gasby means of a catalyst; the diagnostic equipment comprising a firstair-fuel-ratio sensor which detects the air fuel ratio of the exhaustgas upstream of the catalyst; a second air-fuel-ratio sensor whichdetects the air fuel ratio of the exhaust gas downstream of thecatalyst; catalyst-deterioration-index calculation means for calculatinga catalyst deterioration index indicative of a deterioration state ofthe catalyst from output signals of the first air-fuel-ratio sensor andthe second air-fuel-ratio sensor; catalyst-deterioration decision meansendowed with a predetermined threshold value, for deciding thedeterioration state of the catalyst through a comparison between thethreshold value and the catalyst deterioration index; abnormalitydetection means for detecting any abnormality of the engine system asaffects the catalyst deterioration index; and at least one memberselected from the group consisting of catalyst-deterioration-indexcorrection means for correcting the catalyst deterioration index whenthe abnormality has been detected by the abnormality detection means,and catalyst-deterioration-decision interruption means for interruptingthe decision of the catalyst-deterioration decision means when theabnormality has been detected by the abnormality detection means.

[0013] Herein, the abnormality detection means may well comprise misfiredetection means for detecting a combustion state of the engine so as todetect occurrence of misfire of the engine.

[0014] The operation of the first aspect of performance of the presentinvention will be described.

[0015] The catalyst-deterioration-index calculation means calculates acatalyst deterioration index indicative of a deterioration state of thecatalyst from output signals of the first air-fuel-ratio sensor and thesecond air-fuel-ratio sensor. The catalyst-deterioration decision meansdecides the deterioration state of the catalyst through a comparisonbetween the threshold value and the catalyst deterioration index. Thecatalyst-deterioration-index correction means corrects the catalystdeterioration index when the abnormality has been detected by theabnormality detection means, and the catalyst-deterioration-decisioninterruption means interrupts the decision of the catalyst-deteriorationdecision means when the abnormality has been detected by the abnormalitydetection means.

[0016] It is also allowed that the engine system includes a secondaryair system which introduces air into a part of an exhaust pipe locatedbetween the engine and the catalyst, and that the abnormality detectionmeans comprises secondary-air-system failure detection means fordetecting a failure of the secondary air system.

[0017] In the second aspect of performance of the present invention,there is provided a diagnostic equipment for an exhaust gas cleaningapparatus; the exhaust gas cleaning apparatus being directed toward anengine system furnished with an air-fuel-ratio controller which detectsan air fuel ratio of exhaust gas emitted from an engine and whichadjusts a quantity of fuel injection so as to hold the air fuel ratio ofthe exhaust gas at a predetermined value, and cleaning the exhaust gasby means of a catalyst; the diagnostic equipment comprising anair-fuel-ratio sensor which detects the air fuel ratio of the exhaustgas; sensor-deterioration-index calculation means for calculating anair-fuel-ratio-sensor-deterioration index indicative of a deteriorationstate of the air-fuel-ratio sensor from an output signal of theair-fuel-ratio sensor; sensor-deterioration decision means endowed witha predetermined threshold value, for deciding the deterioration state ofthe air-fuel-ratio sensor through a comparison between the thresholdvalue and the air-fuel-ratio-sensor-deterioration index; abnormalitydetection means for detecting any abnormality of the engine system asaffects the air-fuel-ratio-sensor-deterioration index; and at least onemember selected from the group consisting of sensor-deterioration-indexcorrection means for correcting the air-fuel-ratio-sensor-deteriorationindex when the abnormality has been detected by the abnormalitydetection means, and sensor-deterioration-decision interruption meansfor interrupting the decision of the sensor-deterioration decision meanswhen the abnormality has been detected by the abnormality detectionmeans.

[0018] Herein, the abnormality detection means may well comprise misfiredetection means for detecting a combustion state of the engine so as todetect occurrence of misfire of the engine.

[0019] It is also allowed that the engine system includes a secondaryair system which introduces air into a part of an exhaust pipe locatedbetween the engine and the catalyst, and that the abnormality detectionmeans comprises secondary-air-system failure detection means fordetecting a failure of the secondary air system.

[0020] The operation of the second aspect of performance of the presentinvention will be described.

[0021] The sensor-deterioration-index calculation means calculates anair-fuel-ratio-sensor-deterioration index indicative of a deteriorationstate of the air-fuel-ratio sensor from an output signal of theair-fuel-ratio sensor. The sensor-deterioration decision means decidesthe deterioration state of the air-fuel-ratio sensor through acomparison between the threshold value and theair-fuel-ratio-sensor-deterioration index. Thesensor-deterioration-index correction means corrects theair-fuel-ratio-sensor-deterioration index when the abnormality has beendetected by the abnormality detection means. Thesensor-deterioration-decision interruption means interrupts the decisionof the sensor-deterioration decision means when the abnormality has beendetected by the abnormality detection means.

[0022] As stated above, according to the corresponding one of theaspects of performance of the present invention, the degree ofdeterioration of the catalyst or the air-fuel-ratio sensor is noterroneously decided even when the misfire of the engine and the failureor deterioration of the secondary air system has occurred. It istherefore avoided to replace the catalyst or the air-fuel-ratio sensorin spite of no deterioration thereof, or to run the engine in spite ofthe deterioration of the catalyst or the air-fuel-ratio sensor withoutthe replacement thereof. Accordingly, the present invention serves toeliminate wasteful expenses and to prevent air pollution.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]

[0024]FIG. 1 is a block diagram showing the construction of oneembodiment of the present invention;

[0025]FIG. 2 is a diagram showing examples of the output signals ofair-fuel-ratio sensors which are respectively mounted upstream anddownstream of a catalyst;

[0026]FIG. 3 is a graph for explaining the relationship between thedegree of catalyst deterioration and the index of catalystdeterioration;

[0027]FIG. 4A is a graph showing the situation of the output signal ofthe upstream air-fuel-ratio sensor in the case where no misfire arises,while FIG. 4B is a graph showing the situation of the output signal ofthe upstream air-fuel-ratio sensor in the case where misfire arises;

[0028]FIG. 5 is a graph showing influences which the misfire exerts onthe catalyst deterioration index in a case where the downstreamair-fuel-ratio sensor is an O₂-sensor;

[0029]FIG. 6 is a graph showing a correction coefficient and decisioninterruption regions which correspond to the case of FIG. 5;

[0030]FIG. 7 is a graph showing influences which the misfire exerts onthe catalyst deterioration index in a case where the downstreamair-fuel-ratio sensor is a so-called “UEGO sensor(universal exhaust gasoxygen sensor)”;

[0031]FIG. 8 is a graph showing a correction coefficient and a decisioninterruption region which correspond to the case of FIG. 7;

[0032]FIG. 9 is a graph showing a correction coefficient and a decisioninterruption region in the case where the leakage quantity of secondaryair is considered in correspondence with the O₂-sensor in FIG. 5;

[0033]FIG. 10 is a graph showing a correction coefficient and a decisioninterruption region in the case where the leakage quantity of secondaryair is considered in correspondence with the wide-region sensor in FIG.7;

[0034]FIG. 11 is a block diagram showing the construction of anotherembodiment of the present invention;

[0035]FIG. 12 is a graph showing influences which misfire exerts on anindex for the deterioration of an air-fuel-ratio sensor which is mountedupstream of a catalyst in the embodiment depicted in FIG. 11;

[0036]FIG. 13 is a graph showing a correction coefficient and a decisioninterruption region which correspond to the case of FIG. 12; and

[0037]FIG. 14 is a graph showing a correction coefficient and a decisioninterruption region in the case where the leakage quantity of secondaryair is taken into consideration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Now, embodiments of the present invention will be described inconjunction with the accompanying drawings.

[0039]FIG. 1 is a block diagram showing the construction of a diagnosticequipment which is one embodiment of the present invention, togetherwith that of an engine system to which the diagnostic equipment isapplied.

[0040] First, the illustrated example of the engine system on which thepresent invention is premised will be outlined. However, an object towhich the diagnostic equipment of the present invention is to be appliedshall not be restricted to the illustrated example.

[0041] The quantity Qa of air to be introduced into an engine 1 bysuction is measured by an air flow meter 2. In addition, the revolutionspeed or number (revolutions per minute abbreviated to “r. p. m.”) Ne ofthe engine 1 is measured by r. p. m. measurement means not shown.Air-fuel-ratio feedback control means 3 evaluates the basic injectionquantity F_(o) of fuel from the suction air quantity Qa and the r. p. m.Ne in accordance with Equation (1) given below:

F _(o) =k Qa/Ne  (1)

[0042] k: coefficient

[0043] An air-fuel-ratio sensor 7 measures the air fuel ratio of exhaustgas emitted from the engine 1. As stated before, the expression“air-fuel-ratio sensor” in this specification shall cover both the“O₂-sensor” which detects the oxygen concentration in terms of a binaryvalue (and the output of which changes suddenly near the stoichiometricratio), and the so-called “UEGO sensor” which detects the oxygenconcentration linearly. The air-fuel-ratio feedback control means 3obtains a correction coefficient α in dependency on the output of theair-fuel-ratio sensor 7, and evaluates the injection quantity F bycorrecting the basic injection quantity F_(o) in accordance withEquation (2) given below:

F=F _(o)(1+α)  (2)

[0044] Further, the air-fuel-ratio feedback control means 3 applies toan injector 4 a pulse signal whose width corresponds to the injectionquantity F. Thus, it subjects the quantity of fuel feed to a feedbackcontrol. Owing to such control operations, the air fuel ratio of mixtureis held at or near the stoichiometric ratio. A catalyst 5 for decreasingnoxious components contained in the exhaust gas, oxidizes unburnt gascomponents (hydrocarbons abbreviated to “HC”) and carbon monoxide (CO)and simultaneously deoxidizes nitrogen oxides (NO_(x)) Such a catalystis called a “ternary catalyst”. The air fuel ratio needs to be held ator near the stoichiometric ratio in order that the oxidizing anddeoxidizing reactions based on the ternary catalyst may be effected atthe same time. In turn, the air-fuel-ratio feedback control needs to beprecisely executed for keeping the stoichiometric ratio.

[0045] A secondary air system 6 introduces air into an exhaust pipe 100by means of a pump 60 in the operating condition of the engine 1 (forexample, at the start of the engine 1) as requires the establishment ofa state in which the mixture contains the fuel in excess of thestoichiometric ratio (that is, the mixture is “rich”). Thus, thesecondary air system 6 functions to burn and decrease the surplus HCemitted into the exhaust pipe 100. Although not clearly seen from thedrawing, the pump 60 is constructed so as to be operable in associationwith the air-fuel-ratio feedback control means 3, etc.

[0046] Next, the diagnostic equipment of this embodiment will bedescribed.

[0047] As illustrated in FIG. 1, the diagnostic equipment of thisembodiment is constructed comprising misfire detection means 9,secondary-air-system failure detection means 10,catalyst-deterioration-index calculation means 11,catalyst-deterioration decision means 12, catalyst-deterioration-indexcorrection means 13 and catalyst-deterioration-decision interruptionmeans 14.

[0048] The catalyst-deterioration-index calculation means 11 serves todetect the deterioration state of the catalyst 5. Thecatalyst-deterioration-index calculation means 11 in this embodimentdetects the deterioration of the catalyst 5 by utilizing the correlationbetween the signals of the air-fuel-ratio sensor 7 located upstream ofthe catalyst 5 and an air-fuel-ratio sensor 8 located downstream of thesame 5. More specifically, in the state in which the catalyst 5undergoes no deterioration, the output signal of the downstreamair-fuel-ratio sensor 8 does not fluctuate similarly to that of theupstream--air-fuel-ratio sensor 7. However, as the catalyst 5deteriorates, the output signal of the downstream air-fuel-ratio sensor8 comes to exhibit a fluctuation similar to that of the output signal ofthe upstream air-fuel-ratio sensor 7. These facts are utilized for thedetection of the catalyst deterioration. Such a method of detecting thecatalyst deterioration will be outlined below.

[0049] The catalyst-deterioration-index calculation means 11 firstmeasures the output signals of the air-fuel-ratio sensors 7 and 8synchronously. Subsequently, D. C. (direct-current) components whichdisturb the deterioration detection are removed from the measuredsignals by the use of high-pass filters. FIG. 2 illustrates examples ofthese signals. In the figure, symbol x denotes the signal obtained byremoving the D. C. component from the output signal of the upstreamair-fuel-ratio sensor 7, while symbol y denotes the signal obtained byremoving the D. C. component from the output signal of the downstreamair-fuel-ratio sensor 8.

[0050] Besides, the catalyst-deterioration-index calculation means 11calculates the auto-correlation function φxx (Equation (3) given below)of the signal x and the cross-correlation function φxy (Equation (4)) ofthe signals x and y:

φxx(τ)=∫x(t)x(t−τ)dt  (3)

[0051] t: time

[0052] τ: phase

φxy(τ)=∫x(t)y(t−τ)dt  (4)

[0053] t: time

[0054] τ: phase

[0055] Since the auto-correlation function φxx(τ) assumes the maximumvalue φxx(0) for τ=0, the relationship of the following equation (5)holds:

(φxx)_(max) =φxx(0)  (5)

[0056] Further, the maximum value of the cross-correlation functionφxy(τ) is found by varying the phase τ within the integral section ofthis function φxy(τ). Assuming that the maximum value is assumed forτ=τ_(o), the following equation (6) is obtained:

(φxy)_(max) =φxy(τ_(o))  (6)

[0057] The catalyst deterioration index Φc is calculated from thesevalues in accordance with the following equation (7):

Φc=(φxy)_(max)/(φxx)_(max)  (7)

[0058]FIG. 3 illustrates the relationship between a formula φxy(τ)/(φxx)_(max) and the degree of catalyst deterioration. In a casewhere the degree of the deterioration of the catalyst 5 is high (thecatalyst 5 has deteriorated heavily), the degree of the correlationbetween the output signal of the upstream air-fuel-ratio sensor 7 andthat of the downstream air-fuel-ratio sensor 8 is high, and a high peak(maximum value) is demonstrated. On the other hand, in a case where thedegree of the deterioration of the catalyst 5 is low (the catalyst 5 hasdeteriorated lightly), the degree of the correlation between both theoutput signals is low, and only a low peak (maximum value) isdemonstrated. Accordingly, the degree of the deterioration of thecatalyst 5 can be detected in accordance with the magnitude of thecatalyst deterioration index Φc.

[0059] Incidentally, the method of detecting the deterioration degreestated here has already been proposed in Japanese PatentLaid-Open(KOKAI) No. 171924/1993.

[0060] The method of detecting the catalyst deterioration shall not berestricted to the aforementioned method. It is also allowed to employ,for example, a method wherein the catalyst deterioration is detectedfrom the fluctuation widths, phase difference, frequencies or the likeof the output signals of the air-fuel-ratio sensors 7 and 8. A largenumber of other examples have also been known, and any of these methodsmay well be employed. It is necessary, however, to alter a correctionmethod etc. which will be explained below, in correspondence with theemployed detection method.

[0061] The catalyst-deterioration decision means 12 checks thedeterioration state of the catalyst 5 found by thecatalyst-deterioration-index calculation means 11, thereby decidingwhether or not the exhaust gas cleaning apparatus of the engine systemis faulty. In this embodiment, the decision is rendered by comparing thecatalyst deterioration index Φc with a predetermined value setbeforehand (the predetermined value corresponds to a “threshold value”in the appended claims). By way of example, when the catalystdeterioration index Φc is greater than the predetermined value, thefault of the exhaust gas cleaning apparatus (namely, the failure of thecatalyst 5) is decided. In the case of the fault, the driver of avehicle on which the engine system is installed is warned by lighting upan alarm lamp not shown.

[0062] The misfire detection means 9 functions to detect the presence orabsence of misfire every combustion stroke proceeding in the cylinder ofthe engine 1. A method of detecting the misfire is, for example, onedisclosed in the official gazette of Japanese Patent ApplicationLaid-open No. 206342/1991 wherein the misfire is detected from thefluctuation of the revolution speed (r. p. m.) of an engine. This methoddetects an r. p. m. fluctuation waveform or a combustion state duringthe combustion stroke of the engine. Another method of detecting thecombustion state on the basis of the r. p. m. fluctuation is disclosedin U.S. Patent No. 4627399. Still another method detects the combustionstate from a combustion pressure, a temperature or/and the like in thecombustion chamber of the engine, or from the pulsation of the pressureof exhaust gas or the temperature of the exhaust gas. Further, therehave been known a method wherein the combustion state is detected froman ionic current which flows across an ion gap formed in the combustionchamber, as disclosed in U.S. Pat. No. 4648367, a method wherein it isdetected by measuring combustion light in the combustion chamber, and amethod wherein it is detected from the waveform of current flowingthrough an ignition coil, or the like. The misfire detection means 9 inthis embodiment may employ any of such numerous known methods.

[0063] The secondary-air-system failure detection means 10 functions todetect the failure and deterioration of the secondary air system 6. Thismeans 10 is realized by, for example, a method wherein a flow meter isdisposed midway of the air passage of the secondary air system 6 so asto detect the actual flow rate of air and wherein the failure ordeterioration is detected on the basis of the difference between thedetected actual flow rate and a flow rate estimated from the controlledvariable (voltage or current value) of the pump 60. Another method isbased on the correction coefficient α explained before. Morespecifically, in the state in which the secondary air system 6 isoperated, the air-fuel-ratio sensor 7 detects oxygen in a largerquantity. Therefore, when the air-fuel-ratio feedback control explainedbefore is performed as it is, the correction coefficient α enlarges soas to increase the quantity of feed fuel. By exploiting this fact, themethod decides the flow of no secondary air and detects the failure ofthe secondary air system 6 in such a case where the correctioncoefficient α does not enlarge in spite of the actuation of, e. g., thepump 60. The secondary-air-system failure detection means 10 may employany of such numerous known methods.

[0064] The catalyst-deterioration-index correction means 13 corrects thecatalyst deterioration index Φc when the misfire of the engine 1 or thefailure or deterioration of the secondary air system 6 has been detectedby the misfire detection means 9 or by the secondary-air-system failuredetection means 10. Then, the correction means 13 delivers the correctedcatalyst deterioration index to the catalyst-deterioration decisionmeans 12. Incidentally, the correction method will be described latertogether with the operation of this embodiment.

[0065] The catalyst-deterioration-decision interruption means 14 isendowed with a predetermined frequence or extent which concerns themisfire of the engine 1 or the failure and deterioration of thesecondary air system 6 and which serves as a criterion for theinterruption of the catalyst-deterioration decision. Thus, when themisfire or the failure or deterioration detected by the misfiredetection means 9 or the secondary-air-system failure detection means 10exceeds the predetermined frequence or extent, the interruption means 14generates a signal for interrupting the decision of the deterioration ofthe catalyst 5 by the catalyst-deterioration decision means 12. By theway, the frequence or extent may well be altered depending upontemperature etc. as will be explained later.

[0066] There will be described the operation of this embodiment in thecase where the engine 1 misfires.

[0067]FIGS. 4A and 4B illustrate examples of the output signals of theair-fuel-ratio sensor 7 located upstream of the catalyst 5, in anon-misfiring condition and a misfiring condition, respectively.

[0068] When the misfire has occurred, the unburnt HC and the air flowinto the exhaust pipe 100. Therefore, a spike signal S4 which indicatesthat the exhaust gas is lean (in other words, the exhaust gas containsthe larger quantity of oxygen) appears in synchronism with the misfire.In such a misfiring condition, the reactions of oxidizing the unburnt HCproceed together with the ordinary cleaning reactions within thecatalyst 5. Nevertheless, the HC and oxygen components which have notreacted flow downstream of the catalyst 5. In consequence, thedownstream air-fuel-ratio sensor 8 is affected as stated below.

[0069] In the case where the air-fuel-ratio sensor 8 is the O₂-sensorwhich produces the binary output, the output signal shifts to the sidethereof indicating the leaner exhaust gas and also has its amplitudedecreased. As illustrated in FIG. 5, therefore, the catalystdeterioration index Φc changes in correspondence with the frequence inthe misfire and becomes a smaller value. That is, the degree of thecatalyst deterioration is estimated to be lower than the actual one. Inthe catalyst-deterioration-index correction means 13, accordingly, acorrection which enlarges the catalyst deterioration index Φc inaccordance with the frequence in the misfire may be performed bymultiplying the index Φc by a correction coefficient Kc as illustratedin FIG. 6 by way of example. Alternatively, it is allowed to perform acorrection in which the predetermined value to be compared with thecatalyst deterioration index Φc in the catalyst-deterioration decisionmeans 12 is made smaller in accordance with the frequence in the misfirecontrariwise to the above. In such a case where one cylinder continuesto misfire in the engine 1 of, e. g., six cylinders, oxygen and HC whichought to react in proper quantities by the combustion do not entirelyreact within the exhaust pipe 100 as well as the catalyst 5, and hence,the larger quantity of oxygen flows even to the lower stream part of theexhaust pipe 100 with respect to the catalyst 5. In consequence, thedownstream air-fuel-ratio sensor 8 delivers the output signal whichindicates that the air fuel ratio is greater than the stoichiometricratio (in other words, the exhaust gas is leaner than one of thestoichiometric ratio), irrespective of the degree of the deteriorationof the catalyst 5. Moreover, the signal becomes a fixed value (thesignal of substantially null amplitude). As a result, the catalystdeterioration index Φc assumes a fixed value irrespective of the degreeof the catalyst deterioration and cannot be corrected. Accordingly, in acase where the occurring frequence in the misfire is lower than apredetermined value, the catalyst deterioration index Φc shouldpreferably be corrected in accordance with the pertinent frequence. Onthe other hand, in a case where the frequence is higher than thepredetermined value, the decision of the catalyst deterioration shouldpreferably be interrupted while the misfire is occurring or thefrequence is in excess of the predetermined value.

[0070] The magnitude of the correction and the misfire frequencepermitting the correction, change depending upon the temperature of thecatalyst 5, the load of the engine 1, etc. This is based on such areason that the velocities of the oxidizing reactions of the HC differdepending upon the temperature of the catalyst 5 (the reactionvelocities are higher as the temperature is higher within a temperaturerange which the catalyst 5 can usually assume). It is thereforefavorable that the correction coefficient Kc and the misfire frequencefor interrupting the catalyst deterioration decision are altered asexemplified in FIG. 6, depending upon the temperature of the catalyst 5and the operating conditions of the engine 1. Many of the actualmisfiring states of the engine 1 are such that one or more cylindersfail to ignite substantially continuously on account of, e. g., thetrouble of an ignition system. In this case, the misfire at thefrequence not permitting the correction of the catalyst deteriorationindex Φc is detected at one stroke. Accordingly, it is oftensatisfactory that, when the misfire has occurred at the frequenceaffecting the detection of the catalyst deterioration, the decision ofthe catalyst deterioration is interrupted immediately without performingthe index correction. In this case, in many of the known examples, themisfire detection means 9 gives warning to the driver against theoccurrence of the misfire. In practical use, therefore, it hardly posesany problem that the decision of the catalyst deterioration is restartedafter the ignition system, for example, has been repaired on the basisof the warning.

[0071] Meanwhile, in the case where the downstream air-fuel-ratio sensor8 is the so-called “UEGO sensor”, it delivers the output signalindicating the lean exhaust gas in substantial synchronism with thespike output signal of the upstream air-fuel-ratio sensor 7 in themisfiring condition (refer to FIG. 4B). As illustrated in FIG. 7,therefore, when the frequence in the misfire is low, the catalystdeterioration index Φc becomes a value which is greater though slightly.When the frequence in the misfire is somewhat high, the catalyst 5 doesnot react normally in spite of no deterioration thereof, so that theair-fuel-ratio sensors 7 and 8 deliver signal waveforms being closelysimilar to each other, and the catalyst deterioration index Φcdemonstrates a great value. It is accordingly recommended that, asillustrated in FIG. 8 by way of example, the correction of multiplyingthe catalyst deterioration index Φc by the coefficient Kc which makesthis index Φc smaller is performed when the misfire frequence is low,while the decision of the catalyst deterioration is inhibited when themisfire frequence is high.

[0072] As in the foregoing case of the O₂-sensor, it is oftensatisfactory that, when the misfire has occurred at the frequenceaffecting the detection of the catalyst deterioration, the decision ofthe catalyst deterioration is interrupted immediately without performingthe index correction. Further, on this occasion, the catalystdeterioration index Φc becomes a greater value in the decision of thecatalyst deterioration, and hence, the catalyst 5 undergoing nodeterioration might have been erroneously decided as undergoing thedeterioration. It is therefore favorable to discard the last result ofthe decision of the catalyst deterioration.

[0073] The diagnostic equipment may well be so constructed that, unliketo the above, only the correction by the catalyst-deterioration-index(Φc) correction means 13 is done without disposing thecatalyst-deterioration-decision interruption means 14.

[0074] Next, the operation of this embodiment will be describedconcerning the case where the secondary air system 6 has failed ordeteriorated.

[0075] As stated before, the secondary air system 6 serves to introducethe secondary air into the exhaust pipe 100 by means of the pump 60 andto burn and decrease the surplus HC emitted into the exhaust pipe 100,generally in the operating condition in which the mixture must be made“rich” or set at a smaller air fuel ratio (for example, at the start ofthe engine 1). Usually, the secondary air system 6 is not in operationduring the air-fuel-ratio feedback control. In the catalystdeterioration detecting method of this embodiment, accordingly, aspecial problem is such a failure that the secondary air flows out ofthe secondary air system 6 during the sampling of the data by theair-fuel-ratio sensors 7 and 8.

[0076] Influences in the case of the occurrence of such a failure differdepending upon the positional relationship between the air-fuel-ratiosensor 7 and the inlet 62 of the secondary air system 6 to the exhaustpipe 100, and so forth.

[0077] In the case where the air-fuel-ratio sensor 7 is located on thelower stream side of the exhaust pipe 100 with respect to the inlet 62(as illustrated in FIG. 1), it detects oxygen which has flowed in fromthe secondary air system 6, thereby deciding that the exhaust gas is“lean”. Consequently, the air-fuel-ratio feedback control means 3performs the feedback control so as to increase the feed fuel (namely,to bring the mixture to the “rich” side thereof). As a result, thequantity of the fuel enlarges in excess of an amount which correspondsto the oxygen having flowed in from the secondary air system 6.Eventually, the exhaust gas becomes “rich” at the position of thecatalyst 5. Therefore, the output signal of the air-fuel-ratio sensor 8mounted downstream of the catalyst 5 shifts to the “rich” side thereof.

[0078] In this regard, in the case where the downstream air-fuel-ratiosensor 8 is the O₂-sensor of the type which detects the air fuel ratioin terms of the binary value, the detected air fuel ratio deviates fromthe air fuel ratio of the output variation thereof, and the amplitude ofthe output waveform thereof decreases. As in the case of FIG. 5,therefore, the catalyst deterioration index Φc assumes a smaller valuein accordance with the leakage quantity of the secondary air. When theleakage quantity of the secondary air enlarges more, the output signalof the air-fuel-ratio sensor 8 falls into a fixed state (“rich” state).Therefore, the catalyst deterioration index Φc becomes substantiallynull and can no longer be corrected. It is accordingly recommended that,as illustrated in FIG. 9, the catalyst deterioration index Φc ismultiplied by the coefficient Kc for its correction when the leakagequantity of the secondary air is small, while the detection of thecatalyst deterioration is interrupted when the leakage quantity islarge.

[0079] In general, the secondary-air-system failure detection means 10becomes costly for the precise detection of the leakage quantity, sothat the accuracy thereof is not very high. Therefore, it is sometimesthe case that the decision of the catalyst deterioration is alreadyimpossible when the failure of the secondary air system 6 has beendecided. Accordingly, the decision of the deterioration may well beinterrupted at one stroke without performing the index correction.Alternatively, the diagnostic equipment may well be so constructed that,unlike to the above, only the correction by thecatalyst-deterioration-index (Φc) correction means 13 is done withoutdisposing the catalyst-deterioration-decision interruption means 14.

[0080] On the other hand, in the case where the downstreamair-fuel-ratio sensor 8 is the UEGO sensor, the leakage of the secondaryair incurs the fluctuation of the air fuel ratio exceeding the rangethereof in which the catalyst 5 can efficiently carry out the oxidizingand deoxidizing reactions, and it enlarges the amplitude of the outputsignal of the air-fuel-ratio sensor 8. As a result, the catalystdeterioration index Φc assumes a greater value in accordance with theleakage quantity of the secondary air. When the leakage quantity of thesecondary air enlarges more, the output signal of the air-fuel-ratiosensor 8. exceeds the full-scale value thereof irrespective of thedegree of the deterioration of the catalyst 5, and the catalystdeterioration index Φc can no longer be corrected. It is accordinglyrecommended that, as illustrated in FIG. 10, the catalyst deteriorationindex Φc is multiplied by the coefficient Kc for its correction when theleakage quantity from the secondary air system 6 is small, while thedetection of the catalyst deterioration is interrupted when the leakagequantity is large. As in the foregoing case, it is also allowed hereinthat, when the failure of the secondary air system 6 has been decided,the decision of the catalyst deterioration is interrupted at one strokewithout performing the index correction. Further, on this occasion, thecatalyst deterioration index Φc becomes the greater value in thedecision of the catalyst deterioration, and hence, the catalyst 5undergoing no deterioration might have been erroneously decided asundergoing the deterioration. It is therefore favorable to discard thelast result of the decision of the catalyst deterioration. Thediagnostic equipment may well be so constructed that, unlike to theabove, only the correction by the catalyst-deterioration-index (Φc)correction means 13 is done without disposing thecatalyst-deterioration-decision interruption means 14.

[0081] As thus far described, in the case where the leakage quantity ofthe secondary air is small, the catalyst deterioration index Φc iscorrected by the catalyst-deterioration-index correction means 13. Onthis occasion, the directions of the corrections are the opposite incorrespondence with the sorts of the air-fuel-ratio sensor 8. On theother hand, in the case where the leakage quantity of the secondary airis large, the decision of the catalyst deterioration should preferablybe interrupted by the catalyst-deterioration-decision interruption means14.

[0082] The leakage quantity of the secondary air permitting thecorrection changes depending upon the temperature of the catalyst 5,etc. It is therefore favorable that the magnitude of the correction andthe leakage air quantity for interrupting the catalyst deteriorationdecision are altered depending upon the temperature of the catalyst 5or/and the operating conditions of the engine 1. In general, likewise tothe correction ascribable to the misfire, the correction magnitude ismade smaller as the load of the engine 1 is heavier. In addition, theleakage air quantity for interrupting the decision is altered to theside thereof on which the leakage quantity is larger.

[0083] Now, a diagnostic equipment for deciding the deterioration of anair-fuel-ratio sensor will be described as the second embodiment of thepresent invention.

[0084]FIG. 11 generally illustrates the construction of the diagnosticequipment in this embodiment, together with that of an engine system towhich the diagnostic equipment is applied. An engine 1, an air flowmeter 2, air-fuel-ratio sensor 7, etc. are the same as those describedin the first embodiment, respectively.

[0085] The diagnostic equipment is constructed comprising misfiredetection means 9, secondary-air-system failure detection means 10,sensor-deterioration-index calculation means 21, sensor-deteriorationdecision means 22, sensor-deterioration-index correction means 23 andsensor-deterioration-decision interruption means 24.

[0086] The sensor-deterioration-index calculation means 21 serves todetect the deterioration state of the air-fuel-ratio sensor 7. In thisembodiment, the detection of the deterioration state is effected usingthe auto-correlation function of the output signal of the air-fuel-ratiosensor 7. More specifically, the sensor deterioration-index calculationmeans 21 calculates the auto-correlation function φxx(0) on the basis ofthe signal x (refer to FIG. 2) obtained by removing the D. C. componentfrom the output signal of the air-fuel-ratio sensor 7, in the samemanner as in the foregoing case of the catalyst-deterioration-indexcalculation means 11. In case of adopting the function φxx(0) as adeterioration index Φsr which expresses the degree of deterioration ofthe responsivity of the air-fuel-ratio sensor 7, the deterioration indexΦsr assumes a greater value when the air-fuel-ratio sensor 7 undergoesno deterioration, and it assumes a smaller value when the air-fuel-ratiosensor 7 undergoes the deterioration.

[0087] The method of detecting the deterioration of the air-fuel-ratiosensor 7 is not restricted to the above one. There have been known theother examples of detection methods each of which is based on thedifferential value of the output signal of the air-fuel-ratio sensor 7,a response time period required for the output signal to change to theamount of a predetermined voltage, or the frequency of the outputsignal. Any of these methods may well be employed. Merely, a correctionmethod etc. which will be explained below come to differ incorrespondence with the employed detection method.

[0088] The sensor-deterioration decision means 22 compares the sensordeterioration index Φsr with a predetermined value (the predeterminedvalue corresponds to a “threshold value” in the appended claims),thereby deciding the degree of the deterioration of the air-fuel-ratiosensor 7. When the sensor deterioration index Φsr is greater than thepredetermined value, the failure of the sensor 7 is decided. In the caseof the failure, the driver of a vehicle on which the engine system isinstalled is warned by, for example, lighting up an alarm lamp notshown.

[0089] The misfire detection means 9 and the secondary-air-systemfailure detection means 10 are similar to those in the first embodiment,respectively.

[0090] The sensor-deterioration-index correction means 23 corrects thesensor deterioration index Φsr when the misfire of the engine 1 or thefailure or deterioration of a secondary air system 6 has been detectedby the misfire detection means 9 or by the secondary-air-system failuredetection means 10. Then, the correction means 23 delivers the correctedsensor deterioration index to the sensor-deterioration decision means22.

[0091] The sensor-deterioration-decision interruption means 24 isendowed with a predetermined frequence or extent which concerns themisfire of the engine 1 or the failure of the secondary air system 6 andwhich serves as a criterion for the interruption of thesensor-deterioration decision. Thus, when the misfire or the failure ordeterioration detected by the misfire detection means 9 or thesecondary-air-system failure detection means 10 exceeds thepredetermined frequence or extent, the interruption means 24 generates asignal for interrupting the decision of the deterioration of theair-fuel-ratio sensor 7 by the sensor-deterioration decision means 22.

[0092] There will be described the operation of this embodiment in thecase where the engine 1 misfires.

[0093] When the misfire occurs, a spike “lean” signal (S4 in FIG. 4B)appears. Then, the sensor deterioration index Φsr assumes a greatervalue, depending upon the frequence in the occurrence of the misfire(refer to FIG. 12). That is, the responsivity of the air-fuel-ratiosensor 7 is estimated to be better than the actual one, and the degreeof the sensor deterioration is decided to be lower. This holds truewithout regard to the sorts of the air-fuel-ratio sensor 7. In thesensor-deterioration-index correction means 23, accordingly, acorrection which makes the sensor deterioration index Φsr smaller isperformed by multiplying the index Φsr by a correction coefficient Ksrwhich is determined in accordance with the frequence in the misfire(refer to FIG. 13). When the frequence in the misfire is too high, thesensor deterioration index Φsr disperses greatly. Therefore, thedecision of the deterioration of the air-fuel-ratio sensor 7 isinterrupted by the sensor-deterioration-decision interruption means 24.

[0094] Next, the operation of this embodiment will be describedconcerning the case where the secondary air system 6 has failed.

[0095] As in the case of detecting the deterioration of a catalyst 5, aspecial problem is such a failure that the secondary air leaks throughthe secondary air system 6 during the sampling of the data by theair-fuel-ratio sensor 7. Influences in the case of the occurrence ofsuch a failure differ depending upon, e. g., the mixed state of exhaustgas emitted from the engine 1 and the leakage air flowing in from thesecondary air system 6, in that part of an exhaust pipe 100 whichextends between the air-fuel-ratio sensor 7 and the inlet 62 of thesystem 6 to the exhaust pipe 100.

[0096] In this regard, in the case where the air-fuel-ratio sensor 7 isthe O₂-sensor of the type which detects the air fuel ratio in terms ofthe binary value, a signal on the “rich” side thereof becomes difficultof appearing on account of the secondary air (oxygen) leaking in fromthe inlet 62. Therefore, the amplitude of the output waveform of theair-fuel-ratio sensor 7 decreases, and the sensor deterioration indexΦsr assumes a smaller value, in accordance with the leakage quantity ofthe secondary air. When the leakage quantity of the secondary airenlarges more, the sensor deterioration index Φsr becomes substantiallynull and can no longer be corrected. As illustrated in FIG. 14,accordingly, the sensor deterioration index Φsr is multiplied by thecoefficient Ksr for its correction when the leakage quantity of thesecondary air is small, while the detection of the sensor deteriorationis interrupted when the leakage quantity is large.

[0097] On the other hand, in the case where the air-fuel-ratio sensor 7is the UEGO sensor, the sensor deterioration index Φsr assumes a smallervalue subject to the leakage quantity which is small. When the leakagequantity enlarges more, the output of the air-fuel-ratio sensor 7becomes unstable, and the decision of the sensor deterioration can nolonger be rendered. Accordingly, the same correction anddeterioration-decision interruption as in FIG. 14 are done.

[0098] As stated before, in general, the accuracy of thesecondary-air-system failure detection means 10 is not very high.Therefore, it is sometimes the case that the decision of thedeterioration of the air-fuel-ratio sensor 7 is already impossible whenthe failure of the secondary air system 6 has been decided. Accordingly,the decision of the deterioration may well be interrupted at one strokein the case where the failure of the secondary air system 6 has beendecided. Alternatively, the diagnostic equipment may well be soconstructed that, unlike to the above, only the correction by thesensor-deterioration-index correction means 23 is done without disposingthe sensor-deterioration-decision interruption means 24.

[0099] Although the decision of the deterioration of the air-fuel-ratiosensor 7 concerning the responsivity thereof has been explained above,the deterioration of the air-fuel-ratio sensor 7 concerning the outputvoltage thereof may well be decided as another example. In this example,the deterioration is detected by measuring the output of theair-fuel-ratio sensor 7 with the air fuel ratio of the exhaust gas heldat a predetermined value. Herein, the air fuel ratio fluctuates due tothe misfire of the engine 1 or the failure of the secondary air system6, so that the correction of the deterioration index and theinterruption of the deterioration decision are required.

[0100] By way of example, in case of deciding the output voltage of the“rich” side signal of the air-fuel-ratio sensor 7 under the conditionthat the air fuel ratio is held at the predetermined value in the “rich”state of the exhaust gas, the air-fuel-ratio sensor 7 generates a “lean”signal when the secondary air is leaking through the secondary airsystem 6. As a result, the deterioration of the air-fuel-ratio sensor 7is erroneously decided. Besides, a similar situation takes place whenthe engine 1 misfires. In case of a low frequence in the misfire, theair-fuel-ratio sensor 7 delivers a “lean” spike signal (S4 in FIG. 4B).On the other hand, in case of a high frequence in the misfire, theoutput signal of the binary air-fuel-ratio sensor 7 demonstrates a fixedvalue (“lean” state), or the output signal of the UEGO sensor 7disperses greatly. Accordingly, the present invention is also applicableto such a deterioration decision.

[0101] In addition, although the detection of the deterioration of theair-fuel-ratio sensor 7 mounted upstream of the catalyst 5 has beenreferred to in this embodiment, the correction of the deteriorationindex and the interruption of the deterioration decision need to be donein correspondence with the misfire of the engine 1 and the failure ofthe secondary air system 6, also in the detections of the deteriorationsof an air-fuel-ratio sensor mounted downstream of the catalyst 5. Thepresent invention is also applicable to the deterioration detections.

[0102] As thus far explained, the correction of the sensor deteriorationindex Ksr and the interruption of the sensor deterioration decision needto be done in correspondence with the misfire of the engine 1 and thefailure of the secondary air system 6, also in the detection of thedeterioration of the air-fuel-ratio sensor, so that the presentinvention is also applicable to the deterioration detection.

[0103] Further, although the case of correcting the catalyst or sensordeterioration index has been described in the above, it is substantiallythe same to correct the threshold value with which the catalyst orsensor deterioration index is compared in order to decide thedeterioration.

What is claimed is:
 1. A diagnostic equipment for an exhaust gascleaning apparatus; said exhaust gas cleaning apparatus being directedtoward an engine system furnished with an air-fuel-ratio controllerwhich detects an air fuel ratio of exhaust gas emitted from an engineand which adjusts a quantity of fuel injection so as to hold the airfuel ratio of the exhaust gas at a predetermined value, and cleaningsaid exhaust gas by means of a catalyst; said diagnostic equipment,comprising: a first air-fuel-ratio sensor which detects the air fuelratio of the exhaust gas upstream of said catalyst; a secondair-fuel-ratio sensor which detects the air fuel ratio of the exhaustgas downstream of said catalyst; catalyst-deterioration-indexcalculation means for calculating a catalyst deterioration indexindicative of a deterioration state of said catalyst from output signalsof said first air-fuel-ratio sensor and said second air-fuel-ratiosensor; catalyst-deterioration decision means endowed with apredetermined threshold value, for deciding the deterioration state ofsaid catalyst through a comparison between the threshold value and thecatalyst deterioration index; abnormality detection means for detectingany abnormality of said engine system as affects said catalystdeterioration index; and at least one member selected from the groupconsisting of catalyst-deterioration-index correction means forcorrecting said catalyst deterioration index when the abnormality hasbeen detected by said abnormality detection means, andcatalyst-deterioration-decision interruption means for interrupting thedecision of said catalyst-deterioration decision means when theabnormality has been detected by said abnormality detection means.
 2. Adiagnostic equipment for an exhaust gas cleaning apparatus as defined inclaim 1, wherein said abnormality detection means comprises misfiredetection means for detecting a combustion state of said engine so as todetect occurrence of misfire of said engine.
 3. A diagnostic equipmentfor an exhaust gas cleaning apparatus as defined in claim 1, wherein:said engine system includes a secondary air system which introduces airinto a part of an exhaust pipe located between said engine and saidcatalyst; and said abnormality detection means comprisessecondary-air-system failure detection means for detecting a failure ofsaid secondary air system.
 4. A diagnostic equipment for an exhaust gascleaning apparatus; said exhaust gas cleaning apparatus being directedtoward an engine system furnished with an air-fuel-ratio controllerwhich detects an air fuel ratio of exhaust gas emitted from an engineand which adjusts a quantity of fuel injection so as to hold the airfuel ratio of the exhaust gas at a predetermined value, and cleaningsaid exhaust gas by means of a catalyst; said diagnostic equipment,comprising: an air-fuel-ratio sensor which detects the air fuel ratio ofthe exhaust gas; sensor-deterioration-index calculation means forcalculating an air-fuel-ratio-sensor-deterioration index indicative of adeterioration state of said air-fuel-ratio sensor from an output signalof said air-fuel-ratio sensor; sensor-deterioration decision meansendowed with a predetermined threshold value, for deciding thedeterioration state of said air-fuel-ratio sensor through a comparisonbetween the threshold value and the air-fuel-ratio-sensor-deteriorationindex; abnormality detection means for detecting any abnormality of saidengine system as affects said air-fuel-ratio-sensor-deterioration index;and at least one member selected from the group consisting ofsensor-deterioration-index correction means for correcting saidair-fuel-ratio-sensor-deterioration index when the abnormality has beendetected by said abnormality detection means, andsensor-deterioration-decision interruption means for interrupting thedecision of said sensor-deterioration decision means when theabnormality has been detected by said abnormality detection means.
 5. Adiagnostic equipment for an exhaust gas cleaning apparatus as defined inclaim 4, wherein said abnormality detection means comprises misfiredetection means for detecting a combustion state of said engine so as todetect occurrence of misfire of said engine.
 6. A diagnostic equipmentfor an exhaust gas cleaning apparatus as defined in claim 4, wherein:said engine system includes a secondary air system which introduces airinto a part of an exhaust pipe located between said engine and saidcatalyst; and said abnormality detection means comprisessecondary-air-system failure detection means for detecting a failure ofsaid secondary air system.